1. Field of the Invention
This invention relates to oxide-based materials that have one application as phosphors. More particularly, the phosphors are oxides doped with Pr3+ and Mn2+ and exhibit quantum splitting when irradiated with vacuum ultraviolet (xe2x80x9cVUVxe2x80x9d) radiation. This invention also relates to a method of making and rules for designing such quantum-splitting phosphors.
2. Background of the Invention
The conversion of a single ultraviolet (xe2x80x9cUVxe2x80x9d) photon into two visible photons with the result that the quantum efficiency of luminescence exceeds unity is termed quantum splitting. Quantum splitting materials are very desirable for use as phosphors for fighting applications, such as fluorescent lamps. A suitable quantum splitting phosphor can, in principle, produce a significantly brighter fluorescent light source due to higher overall luminous output because it can convert to visible light the part of UV radiation that is not absorbed efficiently by traditional phosphors currently used in commercial fluorescent lamps. Quantum splitting has been demonstrated previously in fluoride- and oxide-based materials. A material comprising 0.1% Pr3+ in a matrix of YF3 has been shown to generate more than one visible photon for every absorbed UV photon when excited with radiation having a wavelength of 185 nm. The measured quantum efficiency of this material was 140%, and thus greatly exceeded unity. However, fluoride-based compounds do not have sufficient stability to permit their use as phosphors in fluorescent lamps because they are known to react with mercury vapor that is used in such lamps to provide the UV radiation and form materials that do not exhibit quantum splitting. In addition, producing fluoride-based materials presents a great practical challenge because it involves the use of large quantities of highly reactive and toxic fluorine-based materials.
The applicants recently disclosed oxide-based quantum splitting materials. U.S. Pat. No. 5,552,082 discloses a lanthanum magnesium borate activated with Pr3+ ion. U.S. Pat. No. 5,571,451 discloses a strontium magnesium aluminate activated with Pr3+ ion and charge compensated with Mg2+ ion. Emission spectra of these materials exhibit a large peak at about 405 nm which is characteristic of quantum splitting. However, these materials still exhibit a considerable emission in the UV wavelength range of less than 350 nm. This part of the emission reduces the overall visible light output that otherwise can be higher. Therefore, it is desirable to provide oxide-based quantum-splitting phosphors that have higher quantum efficiency in the visible range than the prior-art quantum splitting materials. It is also desirable to provide more energy-efficient light sources using quantum-splitting phosphors having higher quantum efficiency. It is further desirable to provide method for making and rules for guiding the design of materials having high quantum-splitting capability.
The present invention provides oxide-based phosphors doped with at least Pr3+ and Mn2+ ions, which phosphors exhibit quantum splitting when irradiated with VUV radiation. VUV radiation as used herein is radiation having wavelength shorter than about 215 nm. The oxide phosphors of the present invention are oxides of aluminum or boron having positive counterions selected from Group IIA and IIIA of the Periodic Table. The phosphors of the present invention may be used in mercury vapor discharge lamps to provide energy-efficient light sources.
In one aspect of the present invention, the oxide-based phosphors are strontium aluminates in which strontium may be partially or completely substituted with calcium and which are doped with at least Pr3+ and Mn2+. Such oxide-based phosphors of the present invention have a composition represented by Sr1xe2x88x92xCaxAl12O19:Pr3+,Mn2+ where 0xe2x89xa6xxe2x89xa61. In this convention, the elements following the colons are the activators in the phosphors.
In another aspect of the present invention, the oxide-based phosphors are also doped with Gd3+0 in addition to Pr3+ and Mn2+.
In still another aspect of the present invention, the oxide-based phosphors are doped with La3+ and Mg2+ ions for charge compensation and for minimization of the number of vacancies in the lattice.
In another aspect of the present invention, the oxide-based phosphors are lanthanum borate activated with Pr3+ and Mn2+ in which lanthanum is partially substituted by gadolinium. In addition, the borate phosphors may be doped with Mg2+. Such borate phosphors of the present invention have compositions represented by La1xe2x88x92xxe2x88x92yxe2x88x92zGdxPryMnzB3O6 where x is in the range from about 0.005 to about 0.99, y is in the range from about 0.005 to about 0.1, z is in the range from about 0.005 to about 0.5, and x+y+z less than 1. Another borate phosphor of the present invention has a formula of La1xe2x88x92xxe2x88x92yGdxPryMg1xe2x88x92zMnzB5O10 where x is in the range from about 0.005 to about 0.995, y is in the range from about 0.005 to about 0.1, z is in the range from about 0.005 to about 0.5, and x+y less than 1.
The present invention also provides a method of making improved quantum-splitting aluminate or borate phosphors. The aluminate phosphors have a formula of Sr1xe2x88x92xCaxAl12O19:Pr3+,Mn2+ or Sr1xe2x88x92xCaxAl12O19:Pr3+,Mn2+,A; where 0xe2x89xa6xxe2x89xa61, and A is selected from the group consisting of Gd3+, La3+, Mg2+, and combinations thereof. The borate phosphors have a formula of La1xe2x88x92xxe2x88x92yxe2x88x92z GdxPryMnzB3O6 where x is in the range from about 0.005 to about 0.99, y is in the range from about 0.005 to about 0.1, z is in the range from about 0.005 to about 0.5, and x+y+z less than 1 or a formula of La1xe2x88x92xxe2x88x92yxe2x88x92zGdxPryMg1xe2x88x92zMnzB5O10 where x is in the range from about 0.005 to about 0.995, y is in the range from about 0.005 to about 0.1, z is in the range from about 0.005 to about 0.5, and x+y less than 1. The method comprises the steps of selecting a desired final composition of the phosphor; mixing together oxygen-containing compounds of praseodymium and manganese, and materials selected from the group consisting of oxygen-containing compounds of strontium, calcium, aluminum, boron, gadolinium, lanthanum, and magnesium so as to achieve the desired final composition; forming a substantially homogeneous mixture of the selected compounds; and firing the substantially homogeneous mixture in a non-oxidizing atmosphere at a temperature and for a time sufficient to result in the desired composition and to maintain the praseodymium ion in the 3+ valence state and manganese ion in the 2+ valence state.